These are the commercial jet engines you see, like the Boeing 777, 747, Airbus A330 and A380s. Though there is a considerable reduction of the engine thrust, at these altitudes the lower drag associated with low air density means a more economical flight with a higher True Air Speed.
Crepitus in the knees may be caused by knee injuries such as meniscus tears. These are fairly common in people who play sports, jog, or run. A meniscus tear can cause crepitus as the joint moves. Chondromalacia patella is a dull ache behind the kneecap, usually caused by overuse or injury. There will be damage to the undersurface cartilage covering the kneecap.
Diarrhea is sometimes caused by an infection in the stomach or intestines. It can also be caused by an inability to digest milk products (lactose intolerance), by problems with the pancreas, or by emotional stress. Bacteria, parasites, fungi, or viruses can cause the infection. Parasites: The parasites cryptosporidium or microsporidium used to cause diarrhea in many people with HIV.